U.S. Pat. No. 4,681,103 discloses a shaft connector configured to connect a shaft that has to be partly inserted through the skull of patient to the skull of the patient. The shaft connector has an adaptor housing that includes external screw thread with which the adaptor housing can be mounted in a hole that has been drilled in the skull of a patient. The shaft connector also includes a shaft bush or probe socket with a swivel ball. The probe socket defines a bush channel that extends along a bush channel axis. A set screw is provided to fixate a shaft within the probe socket. A clamp plate having a central hole with a diameter that is smaller than that of the swivel ball is arranged in the adaptor housing. A locking ring having external screw thread that engages internal screw thread on the adapter housing co-operates with the clamp plate so that the clamp plate can be pressed downward to force the clamp plate against the swivel ball thereby locking it in place between the adaptor housing and the clamp plate. If the locking ring is loose, the probe socket can swivel within the adaptor housing.
WO-98/17191 discloses a similar device. In this case the connection between the adaptor housing and the skull is effected by means of hooks on distal ends of flexible legs that are connectable to the adaptor housing. The hooks engage a hole drilled in the skull on the inside of the skull. The adapator housing can be connected to the skull by tightening a first locking ring having internal screw thread that engages external screw thread on the adaptor housing. A second locking ring with external screw thread that engages internal screw thread can be used to fixate a shaft bush having a swivel ball into a desired directional position in the adaptor housing.